1. Field of the Invention
The invention relates to the sensing of wafer-shaped objects and shelves in a container in which the shelves which are arranged substantially parallel to one another serve to receive the objects and radiation from a radiation source which is reflected at front sides of the objects and at the shelves is directed to optoelectronic sensor elements for recording at least one image by means of an imaging optical device.
2. Description of the Related Art
The sensing of semiconductor wafers in the shelves of the storing containers which are used for transporting and storing the semiconductor wafers as well as holding them in readiness during the manufacturing process grows in importance as the automation of manufacturing increases. Of primary importance in this regard is the achievement of maximum reliability in order to prevent damage to the semiconductor wafers during their automated handling. As a rule, the containers that are used have two openings located opposite to one another. As new technology is introduced, such as the storage and transporting of semiconductor wafers in micro-cleanrooms and the merging of the holding receptacles with their outer covering, the opening of this container is often limited to one side which serves simultaneously for the removal of the semiconductor wafers by means of handling devices.
According to U.S. Pat. No. 4,895,486, it is known to determine the presence of wafer-like objects in a carrier (magazine) and their position relative to a reference plane in the carrier by means of a monitoring device in that a first signal for the presence of such an object is combined with a location signal for the object. The first signal is obtained by an optoelectronic sensor which monitors the space in which the objects can be found. The second signal is formed via a position encoder coupled with a drive for moving the carrier up and down. To determine the reference plane and possible holding space of the objects, the space in the carrier is divided vertically into segments. In addition to a segment serving as reference plane and segments without wafer-like objects, window segments in which objects may be present are defined. An indexing of the carrier is effected in that, after the reference plane in the carrier is detected by measuring techniques, the locations of the window segments are determined by and stored in a computer based on construction specifications of the respective carrier being used.
The solution described in Patent DE 4306957 C1 shows a bundle of measurement beams which is emitted by a transmitter and whose center beam lies in the reference plane. This measurement beam bundle is guided between oppositely located walls containing the magazine shelves and is directed to and interrupted by projections of one of the walls, which projections face toward the interior of the magazine and form shelves serving as a support for the disk-shaped objects. As a result of the vertical adjustment in the direction of the magazine shelves which are located one above the other and which accordingly, one after the other, occupy a position in common with the reference plane, an image of the magazine shelves and of the disk-shaped objects located therein is generated by a modulation of the measurement beam bundle.
DE 42 38 834 A1 describes an arrangement which, in addition to a determined construction of a robot for moving semiconductor wafers, also contains, among other things, a sensor arrangement for sensing the presence of these semiconductor wafers in their holding containers. A number of photoreceivers are arranged opposite the holding container in such a way that the photoreceivers are in a spatial relationship to the shelves of the holding containers. An illumination device which is attached to the robot can be moved by the latter into a coordinate relative to the holding container. The signal generated in the photoreceivers during this movement makes it possible to determine the presence of a semiconductor wafer in a shelf allocated to the photoreceiver.
All of the technical solutions mentioned above have the disadvantage that they do not work when used with the above-mentioned containers that are open on one side, since the radiation cannot pass through the holding container. A further disadvantage consists in that a sensing or scanning relative motion is required between the sensor and the substrate. This leads to an increase in the time required for sensing and also to a deterioration of clean room conditions. The latter becomes particularly problematic when the relative movement can only be generated by an adjustment or displacement of the sensor.
U.S. Pat. No. 5,418,382 uses radiation elements which are arranged in a row adjacent to the front sides of the semiconductor wafers. Rod-shaped light waveguides, whose inlet openings are adjacent to the front sides of the semiconductor wafers and whose light outlet openings are adjacent to receiver elements, transmit the reflected light. The described device is limited to detection of substrates in predefined zones and accordingly affords the possibility of detecting a plurality of objects in such a zone; however, it is not possible to detect substrates which are located in different shelves and are accordingly tilted relative to a handling plane.
Due to the fact that it is required to arrange the device in the immediate vicinity of the substrate to be detected, application is limited to containers which are open on two sides as in the other prior art solutions. When containers are used which are open on only one side, the entire device must be removed from the handling area so as to prohibit increased particle contamination.
Beyond this, the use of the device in the immediate vicinity of the substrate impedes the use of SMIF technology and leads to restrictions in automation. The physically large extension of the sensor over the entire height of the container impedes handling of the cassettes with respect to the arrangement directly opposite the substrates and limits the degrees of freedom of possible movements.
The subject matter of DE 195 35 871 A1 also allows the use of magazines or magazine-like containers which are closed on all sides except in the charging direction, but also requires a relative movement. Diffuse scatter light which is formed after a measurement beam bundle impinges on the edge of an object or on an object support is received by a position-sensitive photoreceiver that is usually combined with a sensor in a mechanical constructional unit, and this diffuse scatter light is transformed by means of an electronic amplifier into an analog signal whose value is dependent on the distance between the transmitter and point of incidence. By means of vertical adjustment in the direction of the magazine shelves which are located one above the other, an image of the magazine shelves and of the wafer-shaped objects located in the magazine shelves is generated by means of an amplitude modulation of the output signal brought about by the change in distance between the transmitter and the reflecting object in the plane vertical to the movement direction.